1. Field of the Invention
The present invention relates generally to a wavelength division multiplexer and demultiplexer for use in the field of optical communication and optical information processing, and more particularly to an athermalized wavelength division multiplexer and demultiplexer and method of manufacturing.
2. Technical Background
Computer and communication systems place an ever-increasing demand upon communication link bandwidths. It is generally known that optical fibers offer a much higher bandwidth than conventional coaxial links. Further, a single optical channel in a fiber waveguide uses a small fraction of the available bandwidth of the fiber. In wavelength division multiplexed (WDM) optical communication systems, multiple optical wavelength carriers transmit independent communication channels along a single optical fiber. By transmitting several channels at different wavelengths into one fiber, the bandwidth capability of an optical fiber is efficiently utilized.
Fiber-optic multiplexing and demultiplexing have been accomplished for nearly a decade using a plurality of closely spaced waveguides communicating with an input coupler. The output of the coupler communicates with a second coupler via an optical grating consisting of an array of optical waveguides each of which differing in length with respect to its nearest neighbor by a predetermined fixed amount. The outputs of the second coupler form the outputs of the multiplexing and demultiplexing device. In operation, when a plurality of separate and distinct wavelengths are applied to separate and distinct input ports of the device, they are combined and are transmitted to an output port. The same device may also perform a demultiplexing function in which a plurality of input wavelengths are directed to a predetermined one of the input ports of the apparatus, and each of the input wavelengths is separated from the other and directed to predetermined ones of the output ports.
The grating located between the couplers consists of a plurality of waveguides of different lengths ordered in an array. Wavelength division multiplexers and demultiplexers require precise control of the optical path difference between adjacent waveguides. The optical path difference is the product of the effective index of refraction of the fundamental mode in the waveguide and the physical path difference between adjacent waveguides. The effective index of refraction of the fundamental mode in the waveguides and the physical path differences between adjacent waveguides for currently available wavelength division multiplexers and demultiplexers are typically both temperature dependent. In conventional integrated optical multiplexer and demultiplexer devices, the medium forming the arrayed waveguides has a noticeable temperature dependency which results in changes in the central transmission wavelength which may exceed the transmission bandwidth. As a result, temperature variations in the usually operating temperature range (from about 0xc2x0 C. to about 70xc2x0 C.) induce a wavelength shift which is unacceptable in comparison to the typical accuracy requirements (about 0.1 nm) in center channel position. Consequently, available multiplexer/demultiplexer optical devices of the phased-array type are generally operated in a temperature controlled environment. Typically, control circuits with heating elements are provided to insure a stable temperature environment. However, the use of heating elements to achieve active athermalization is undesirable because it increases the overall cost, size and complexity of the device, and may consume considerable power.
In the case of conventional wavelength division multiplexers having a phased-array optical grating comprised of a plurality of silica waveguides and silica cladding, the variation of channel wavelength as a function of temperature predominately depends on the positive variation of the effective index of refraction of the waveguides as a function of temperature. In an effort to compensate for the positive variation of refractive index as a function of temperature for silica-based materials, polymer overcladding materials having a negative variation of refractive index as a function of temperature have been employed. However, a problem with this arrangement is that as the temperature varies, the difference in refractive index between the core and the cladding varies, and in the worst case, light may not be able to be guided into the waveguide. As a result, optical multiplexer/demultiplexer devices having a phased-array type grating with a polymer overcladding may not be suitable for use over a wide range of ambient temperatures. Another problem with this optical fiber structure is that the polymer overcladding makes it more difficult to connect optical fibers to the input ports of the device.
Another proposed design for maintaining a relatively constant optical path difference between adjacent waveguides in a phased-array involves localizing a polymer in a triangular groove in the phased-array. The groove is etched in the center of the phased-array to the bottom of the waveguides and is filled with a polymer, typically a silicone polymer. The ratio of the optical path difference between adjacent waveguides in the silica region to the optical path difference in the groove can be selected to cancel, or at least minimize, the variation in the mean channel wavelength as a function of temperature. An advantage of the groove design as compared with the overclad design is that the polymer is localized in the middle of the device. This avoids the problem associated with connecting polymer overcladding optical fibers to a device. However, phased-array devices having a polymer filled triangular groove exhibit a loss of about 2 dB in excess of standard phased-array devices. The excess loss is believed to be attributable to free-space propagation of light into the groove. Light is guided into the input side of the waveguides of the phased-array, propagates freely in the groove, and is only partially collected by the output waveguides of the phased-array. The estimated excess loss for such a waveguide increases as a function of the path length in the groove which is not constant, but depends on the number of waveguides in the phased-array. Thus, the loss in the different waveguides is not constant and cross talk may result.
Therefore, there remains a need for optical multiplexer/demultiplexer devices of the phased-array type in which the optical path difference between adjacent waveguides in the phased-array region is more precisely controlled to minimize wavelength shifts to an acceptable level while also minimizing power loss in the transmitted signals without the use of active temperature control means, such as heating elements.
This invention is directed to a passively athermalized optical waveguide device which is useful for optical waveguide division multiplexing and/or demultiplexing, in which the positive variation of the effective index of refraction as a function of temperature in a waveguide having a silica core is compensated by a negative variation in effective index of refraction as a function of temperature in a polymer waveguide core, without unacceptable loss of optical signal power.
In accordance with an aspect of the invention, an optical waveguide device includes an optical phased-array comprising a plurality of curved waveguide cores of different lengths supported on a planar substrate in which each waveguide core includes a first silica segment, a second silica segment, and a central polymer segment connecting the first silica segment with the second silica segment to form a continuous waveguide core. The ratio of the optical path difference between each pair of adjacent waveguide cores in the silica segments to the optical path difference between the adjacent waveguide cores in the polymer segments is selected to minimize variation in the overall optical path difference of the waveguides. The polymer segments of the waveguide cores have a negative variation in effective index of refraction as a function of temperature to compensate for the positive variation in the index of refraction of the silica waveguide core segments as a function of temperature, thereby inhibiting shifting of channel wavelengths due to variations in operating temperature within a predetermined operating temperature range.
In another aspect of the invention, a method of making a passively athermalized optical waveguide device is provided. A planar substrate is provided, and a plurality of adjacent curved silica waveguide cores of different lengths are formed on the planar substrate. The silica waveguide cores are overcladded with a glass overclad to define an optical phased-array. At least one triangular groove is etched into a central portion of the optical phased-array through to the planar substrate to divide each waveguide core into a first silica waveguide core segment and a second silica waveguide core segment, in which the first and second silica waveguide core segments are separated by free space. A plurality of distinct polymer waveguide core segments are formed in the triangular groove, with each polymer waveguide core segment connecting a first silica waveguide core segment with a corresponding second silica waveguide core segment to form a continuous waveguide core. The polymer waveguide core segments may be overcladded to form an optical phased-array comprising a plurality of adjacent waveguides. The dimensions of the groove or grooves which define the optical path lengths of the polymer waveguide core segments, are selected so that the ratio of the optical path difference between each pair of adjacent waveguide cores in the silica segments to the optical path difference between the adjacent waveguide cores in the polymer segments minimizes variation in the overall optical path difference of the adjacent waveguide cores.